Plant tomato seedlings outdoors after the last frost date in your area and when soil temperature reaches 60°F

The single most important rule: tomato seedlings go outside only after your region's last spring frost has passed. Frost kills tomato plants. You can find your last frost date by searching "[your city] last frost date" or checking the USDA Hardiness Zone Map for your zip code. That date is your earliest safe window.

But frost date alone is not enough. Soil temperature matters as much as air temperature. Tomato roots grow slowly in cold soil and the plant will sit stunted rather than establish itself. Wait until soil temperature reaches at least 60°F, ideally 65°F or warmer. You can measure soil temperature with a straightforward soil thermometer pushed 4 inches into the ground in the morning. If you do not have a thermometer, wait one to two weeks after your last frost date—soil usually warms by then.

Seedlings also need to be hardened off before moving outside. This means gradually exposing them to outdoor conditions over 7 to 10 days so they adjust to wind, direct sun, and temperature swings. Start by placing them in a sheltered spot for 2 to 3 hours, then increase time and exposure each day until they can handle a full day and night outdoors.

Key Takeaways

  • Wait until after your region's last frost date—check your USDA zone or search "[your city] last frost date" to find the exact date for your area.
  • Soil temperature must reach 60°F minimum, ideally 65°F or higher, which you can measure with a soil thermometer or estimate by waiting 1 to 2 weeks after the frost date passes.
  • Harden off seedlings over 7 to 10 days by gradually moving them outdoors for increasing lengths of time before planting them in the ground.
  • Seedlings planted too early in cold soil will stall and may rot; those planted after hardening off establish faster and produce earlier.
  • If a late frost is forecast after you plant, cover seedlings with row cloth or move pots back indoors for the night.

How to read your last frost date and why it matters

Your last frost date is the average date of the final frost in spring for your location. After that date, nighttime temperatures typically stay above 32°F. Tomatoes cannot survive frost—even a light frost at 32°F will blacken leaves and kill seedlings. Planting before this date is the most common reason seedlings fail.

To find your last frost date, search online for "[your city] last frost date" or visit the USDA Plant Hardiness Zone Map website and enter your zip code. The map will show your zone and a link to frost dates. Write the date down. If you live in a region with variable weather or near a lake or mountain, the date can shift by a week or two, so ask a local garden center or experienced gardener in your area for their experience.

Once you know the date, count backward 6 to 8 weeks to determine when to start seeds indoors. If your last frost date is May 15, start seeds indoors around March 15 to April 1. This timing ensures seedlings are the right size—6 to 8 inches tall with 2 to 3 sets of true leaves—when it is safe to plant them outside.

Checking soil temperature before planting

Air temperature and soil temperature are not the same. A warm day can fool you into thinking soil is ready when it is still cold. Tomato roots grow best in soil warmer than 60°F. Below that, they sit dormant and the plant does not establish. A soil thermometer costs $10 to $20 and takes the guesswork out.

To measure soil temperature, push the thermometer 4 inches into the ground in the morning, wait 2 to 3 minutes, and read the temperature. Do this in the spot where you plan to plant. If the soil is below 60°F, wait another week and test again. Soil warms fastest in raised beds and containers, slower in in-ground gardens shaded by trees or buildings.

If you do not have a thermometer, a practical rule is to wait 1 to 2 weeks after your last frost date. By then, soil has usually warmed enough. You can also feel the soil—if it feels cold and damp to the touch, it is too cold. Warm soil feels warm and crumbly.

Hardening off seedlings step by step

Seedlings grown indoors under lights are tender. Moving them straight into outdoor sun, wind, and temperature swings causes wilting, leaf burn, and sometimes death. Hardening off gradually toughens the plant over 7 to 10 days.

Day 1 to 2: Place seedlings in a sheltered spot outdoors—a porch, against a wall, or under a tree—for 2 to 3 hours in the morning. Water them before moving them outside. Bring them back in.

Day 3 to 4: Move them to the same spot for 4 to 5 hours, including some direct morning sun. Water if the soil surface feels dry.

Day 5 to 6: Leave them out for 6 to 8 hours, with 2 to 3 hours of direct sun. They may wilt slightly by late afternoon—this is normal. Water in the evening.

Day 7 to 9: Leave them out all day and bring them in at dusk. Increase direct sun exposure to 4 to 6 hours. Water in the morning and evening if needed.

Day 10: Leave them out overnight if nighttime temperatures stay above 50°F. If temperatures drop below 50°F, bring them in. Once they survive a full day and night outdoors, they are ready to plant in the ground.

Common mistakes that delay or damage seedlings

Planting too early is the most common error. Seedlings in cold soil do not grow—they sit there for weeks, become root-bound, and never catch up. A seedling planted at the right time in warm soil will outpace one planted early in cold soil by several weeks.

Skipping hardening off causes leaf scald (white or brown patches on leaves) and wilting. Seedlings need time to build thicker leaf skin and stronger stems. Even if they survive without hardening off, they recover slowly and produce later.

Planting seedlings too deep buries the stem and can cause rot at the soil line. Plant at the same depth they were growing in the pot, or slightly deeper if the stem is leggy—tomatoes can root along the buried stem. Do not bury the lowest leaves.

Watering too much after planting causes root rot in cool soil. Water seedlings after planting, then wait until the top inch of soil dries before watering again. In cool weather, soil stays wet longer.

What to do if frost threatens after planting

Even after you plant, a late frost can arrive. Check the forecast 7 to 10 days ahead. If frost is predicted, you have options.

For seedlings in pots, move them back indoors or into a garage, shed, or cold frame for the night. For seedlings planted in the ground, cover them with row cloth, old sheets, or newspaper held down with stakes or soil. Remove the cover in the morning once temperatures rise above 40°F. Do not leave covers on during the day—heat builds up underneath and can damage plants.

If seedlings are damaged by frost, do not pull them out when ready. Wait a few days. Many recover and send out new growth. If a seedling is completely blackened and does not recover within a week, replace it.

Planting depth and spacing in the garden

Dig a hole slightly larger than the seedling's root ball. If the seedling is leggy (tall and thin), bury the stem up to the lowest set of leaves—tomatoes will root along the buried stem and develop a stronger root system. If the seedling is stocky and the right height, plant at the same depth it was growing in the pot.

Space seedlings 24 to 36 inches apart, depending on the variety. Determinate (bush) types need less space; indeterminate (vining) types need more. Wider spacing improves air flow and reduces disease. Gently firm soil around the seedling and water deeply.

Add a 2-inch layer of mulch around the base, keeping it 2 inches away from the stem. Mulch keeps soil warm, retains moisture, and prevents soil-borne disease from splashing onto leaves.

Frequently Asked Questions

Can I plant seedlings if the forecast shows one warm week after the last frost date?

No. One warm week does not mean frost will not return. Frost can arrive weeks after the official last frost date, especially in spring. Wait until the date has passed and soil is warm. If you plant early and frost hits, you lose the seedlings.

What if my seedlings are already too tall and leggy indoors?

Bury the leggy stem up to the lowest leaves when you plant outdoors. Tomatoes root along the buried stem, creating a stronger root system. The plant will catch up to a shorter, stockier seedling within a few weeks.

How do I know if soil is warm enough without a thermometer?

Wait 1 to 2 weeks after your last frost date, or feel the soil—warm soil feels warm and crumbly to the touch, not cold and damp. If you are unsure, wait another week. Planting in cold soil is worse than planting late.

Do I need to harden off seedlings if I am moving them from a greenhouse to the garden?

Yes. Greenhouse seedlings are also tender, even if they have been in a warm space. Harden them off the same way—gradually expose them to outdoor sun, wind, and temperature swings over 7 to 10 days.

What temperature is too cold to leave seedlings outside overnight?

Anything below 50°F is risky. Bring seedlings indoors if the forecast shows temperatures below 50°F at night. Once nighttime temperatures stay consistently above 50°F, seedlings can stay out overnight.